In a typical cellular radio system, wireless terminals communicate via a radio access network, RAN, to one or more core networks. The wireless terminals can be mobile stations or user equipment, UE, such as portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network, e.g., mobile telephones and laptops with wireless capability.
The RAN covers a geographical area which is divided into cell areas, with each cell area or group of cell areas being served by a radio access node. A cell is a geographical area where radio coverage is provided by equipment at the radio access node. Each cell is identified by an identity within the local radio area. The radio access nodes communicate over the air interface with UEs within the cells served by the node.
In order to maintain a radio connection with the network, the user equipments are handed over from one cell to the next when travelling through the geographical area. To facilitate handovers in cellular systems, a neighbor cell list is used. The neighbor cell list is stored in a neighbor cell list database holding records for each cell in the system. Each record is associated to a cell by a cell identity. Each neighbor cell list contains a set of cells that user equipments within the current cell may be in the range of from a handover perspective. A typical implementation has one master database while other entities, e. g., a radio base station in a radio access node and the user equipment maintains a copy of the relevant records for their context.
A purpose of neighbor cell lists is to provide connecting user equipment with a short list of alternative cells that may provide better signal or performance than the current cell. A typical use is to prepare for handover, when the user equipment moves over a geographical area larger than that the coverage area of a single cell. By limiting the number of cells in the neighbor cell list, the user equipment measurement and processing task may be simplified. The neighbor cell list may also include a set of rules for neighbor cell relations that govern the handovers, for example thresholds, restrictions or timers. Thus, the existence and correctness of the neighbor cell list is essential for a satisfying performance at handover in a cellular radio system.
The handover process involves measurements of performance, e.g., signal strength measurements, for both the serving cell that the UE is connected to and for one or more neighbor cell(s). In the handover decision, it is determined that the UE should add a neighbor cell as serving cell and may disconnect from the current serving cell. A typical handover takes place in the overlapping area between two cells when a mobile is moving from a first to a second cell.
Defining neighboring cells in a proper way may be problematic, in particular when one or more neighboring cells may be moving or nomadic cells. Since the cellular radio networks are constantly growing, moving cells is an ever ongoing process; not only adding new cells to existing neighbor cell lists, but also removing cells that are no longer relevant. In prior art solutions, adding new relations can be a rather slow process, where the system needs to judge additions and removal of cell relations over long periods of time before making a decision.
When a new cell is introduced in a wireless network, it may cause significant effects on the surrounding cells' performance if not all necessary network configuration changes are made at the same time.
Such simultaneousness requires operator planning. Poorly maintained neighbor cell lists affect performance in cellular networks.
Current cellular systems and standards can support nomadic and moving radio base stations, cells that move around between different locations. Typical examples are airplanes and ships, where an onboard radio access node, i.e., a radio base station, provides local coverage onboard. Connection to “the rest of the world” is achieved by satellite and microwave links. Current state of the art depends on separating the moving cell(s) from the fixed cells in the surrounding cellular systems. This is performed by a combination of separated networks, requiring subscribers to enter the cells as roamers and physical shielding to avoid overlap with surrounding cells. Thus, the moving cell is basically isolated from the surroundings and handover between the moving cell and neighboring cells is not contemplated.
However, there is an increasing need to allow moving/nomadic cells as true parts of the cellular network.